This invention relates to a process for recovering valuable metals such as iron, zinc, and the like from an iron dust using a shaft furnace.
The iron dust includes all kinds of iron making dusts generated during an iron manufacturing process using a blast furnace, an LD converter, an electric steel making furnace, a ferroalloy making furnace, and the like. The iron dust also includes a dry dust which is collected by means of a back filter or the like, and a wet dust such as a sludge or the like which is collected by a wet type dust collector.
In general, the iron dust contains a large quantity of valuable metal oxides such as zinc oxide, iron oxide and the like. A method for recovering these valuable metals is already developed. This recovering method is broadly classified into two; one is a method using a rotary kiln and the other is a method using an electric kiln furnace.
The method using the rotary kiln will be described first. In the recovering method disclosed in Japanese Patent Publication No. Sho 59-20734, an iron dust is supplied into a rotary kiln together with a coke as a reducing agent, and valuable metal oxides are reduced. Although zinc, lead, cadmium, etc. have such high boiling points as about 2000.degree. C. in their oxide states, respectively, their boiling points are lowered to about 1000.degree. C. when they are in the reduced states. Zinc, lead and cadmium reduced in the rotary kiln are volatilized, and reoxidized by air within the rotary kiln. Those oxides of zinc, lead and cadmium, i.e., crude zinc oxides, are discharged from the rotary kiln together with exhaust gas and then recovered through a back filter. A non-volatile matter chiefly composed of a metal iron obtained by reducing the iron dust is recovered in the form of an iron clinker from an exhaust end of the rotary kiln. In a method disclosed in Japanese Patent Publication No. Sho 64-4572, an aluminum sludge is used as a reducing agent which is to be supplied into the rotary kiln.
In the recovering method using the rotary kiln as mentioned, there are the following drawbacks. That is, the iron dust is a fine powder having a grain size of about 1 to 10 .mu.m. It is softened by heat of 1000.degree. C. or higher, increased in adhesion strength and adhered to an inner wall surface of the rotary kiln. It requires much time and labor to remove the iron dust thus adhered. Furthermore, since the iron clinker has so small iron content as 50 to 60% and in addition, the iron content is varied significantly, it is not used as an iron material. As a result, the iron clinker is left as a secondary industrial exhaust.
Next, the method using an electric furnace will be described. In the method disclosed in U.S. Pat. No. 4,612,041 (Japanese counterparts: Patent Publication Nos. Sho 63-48934 and Hei 2-13017), an iron dust is agglomerated and this agglomerated matter is dried and preheated in a shaft furnace. Furthermore, in this shaft furnace, hydroxides, carbonates, etc. contained in the agglomerated iron dust are removed and a carbon contained in the iron dust is also burnt and removed. Such pretreated agglomerated matter is supplied into an induction furnace together with coke and reduced here. Zinc among components of the agglomerated iron dust is evaporated and recovered in the form of a metal zinc. Iron, lead and slag are molten in the induction furnace, and separated and recovered. In the shaft furnace, it is also possible that the iron oxide is reduced by adjusting the concentration of CO gas while restraining the reduction of the zinc oxide. Also in this case, the pretreated lump is supplied into the induction furnace together with the coke, and valuable metals are recovered in the same manner as above.
In the method disclosed in U.S. Pat. No. 4,612,041, since a large amount of electric energy is consumed in order to recover valuable metals using the induction furnace, the recovering cost becomes high. In this method, the iron dust is not reduced in the shaft furnace using the coke.